1. Field of the Invention
This invention relates to photothermographic materials that form color images upon light exposure and heat development. More specifically, this invention relates to chromogenic leuco redox-dye-releasing ("RDR") compounds that are suitable for use in photothermographic imaging systems.
2. Background Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (a) a photosensitive material that generates elemental silver when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent for the non-photosensitive, reducible silver source; and (d) a binder. The photosensitive material is generally photographic silver halide that must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. For example, catalytic proximity can be accomplished by partial metathasis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e., the latent image. This produces a black and white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion. One method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion.
A number of methods have been proposed for obtaining color images with dry silver systems. Such methods include, for example, incorporating dye-forming coupler materials into the dry silver systems. For example, known color-forming dry silver systems include: a combination of silver benzotriazole, a magenta, yellow, or cyan dye-forming coupler, an aminophenol developing agent, a base release agent such as guanidinium trichloroacetate, and silver bromide in poly(vinyl butyral); and a combination of silver bromoiodide, sulfonamidophenol reducing agent, silver behenate, poly(vinyl butyral), an amine such as n-octadecylamine, and 2-equivalent or 4-equivalent yellow, magenta or cyan dye-forming couplers.
U.S. Pat. No. 4,021,240 discloses the use of sulfonamidophenol reducing agents and four equivalent photographic color couplers in photothermographic emulsions to produce dye images. U.S. Pat. No. 3,531,286 discloses the use of photographic phenolic or active methylene color couplers in photothermographic emulsions containing p-phenylenediamine developing agents to produce dye images. U.S. Pat. No. 4,463,079 discloses the use of sulfonamidophenol and sulfonamidonaphthol redox-dye-releasing compounds which release a diffusible dye on heat development. U.S. Pat. No. 4,474,867 discloses the use of dye-releasing couplers which, in combination with a reducing agent, release a diffusible dye on heat development. U.S. Pat. No. 4,981,775 discloses the use of redox-dye-releasing compounds, e.g., oxazines, thiazines, and azines, that release a diffusible dye on heat development.
Color images can also be formed by incorporation of leuco dyes into the emulsion. A leuco dye is the reduced form of a color-bearing dye. It is generally colorless or very lightly colored. Upon imaging, the leuco dye is oxidized, and a color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced silver image can be produced. U.S. Pat. No. 4,022,617 discloses the use of leuco dyes in photothermographic emulsions. The leuco dyes are oxidized to form a color image during the heat development of the photothermographic element. Chromogenic leuco dyes having various protecting groups are described in Applicants' Assignee's copending application Ser. No. 07/939,093 (filed Sep. 2, 1992 now allowed) and Ser. No. 08/161,900 (filed Dec. 3, 1993).
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic elements having at least two or three distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4.021.240 and 4,460,681. Various methods to produce dye images and multicolor images with leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963. Various other dye-releasing systems have been disclosed in U.S. Pat. Nos. 4,060,420; 4,731,321; 4,088,496; 4,511,650; and 4,499,180.
It is an object of the present invention to provide alternative heat developable color photographic materials capable of releasing dyes to provide clear, stable color images.